Concurrent delay is one of the most contested areas in construction law, and under NEC4, it's handled very differently from JCT or FIDIC. If you're managing an NEC4 contract and your project is in delay, understanding exactly how NEC4 treats concurrent causation determines whether your extension of time claim succeeds, gets reduced, or collapses entirely.
This guide covers the definition, the contractual mechanism, float ownership, programme analysis, and the practical steps your commercial team should take before a concurrent delay dispute reaches adjudication.
What Is Concurrent Delay?
Concurrent delay arises when two or more independent causes of delay, one attributed to the client (or a compensation event), one attributed to the contractor, are operating simultaneously, and each would on its own have caused delay to completion.
The definition sounds simple. In practice, it's a mess. Courts and adjudicators have spent decades arguing about whether "true concurrency" requires exact calendar overlap, whether delays on different activities count, and what the appropriate remedy should be when you can't cleanly separate the causes.
The Society of Construction Law Delay and Disruption Protocol (2nd edition, 2017) defines concurrent delay as: "a period of project overrun which is caused by two or more effective causes of delay which are of approximately equal causative potency." That phrase, "approximately equal causative potency," does a lot of heavy lifting, and it's almost never straightforward to establish on a live project.
What matters for NEC4 teams is this: the contract doesn't use the phrase "concurrent delay" at all. It never mentions it. Instead, the ECC handles the underlying situation through its compensation event mechanism, programme obligations, and float rules, and the outcomes are often different from what practitioners expect when they arrive from a JCT background.
How NEC4 Handles Concurrent Delay Differently
Under JCT, concurrent delay typically results in the contractor getting an extension of time (because the employer risk event is a ground for EOT) but no loss and expense (because the contractor's own default contributed equally). That's the "extension but no money" outcome most commercial managers know.
NEC4 doesn't work that way.
The Compensation Event Mechanism
Under NEC4, an extension of time is not granted as a stand-alone right. Time is dealt with as part of a compensation event assessment under Clause 60. The Contractor notifies a compensation event, submits a quotation under Clause 62 showing both the time and cost impact, and the Project Manager accepts, rejects, or assesses it.
The key question in a concurrent delay scenario is: does the compensation event cause delay to the Completion Date? Under Clause 63.3, a compensation event assessment includes a delay to the Completion Date only if the event affects the critical path of the Accepted Programme. If the contractor's own delay has already pushed a particular sequence off critical path, the Client risk event may have no demonstrable effect on Completion, even if the contractor is genuinely suffering its impact.
This is where the absence of an Accepted Programme becomes financially catastrophic in concurrent delay situations. Without an accepted, updated programme showing the critical path at the time of the event, there's no baseline against which to measure the event's impact. The Project Manager assesses using their own assumptions under Clause 63.1, and those assumptions will almost always attribute less delay to the compensation event than the contractor believes is fair.
Float Belongs to the Project
This is the single most important difference between NEC4 and most other standard forms.
Under Clause 63.3, a compensation event is assessed as the difference between the Accepted Programme (including any terminal float) and the revised programme incorporating the event. The contract is silent on who "owns" float in the traditional sense, because in NEC4, the question barely arises.
Here's why. NEC4 requires the Accepted Programme to show all float (Clause 31.2). Total float is visible to both parties. When a compensation event is assessed, it's assessed against the programme as it stands. If there was 3 weeks of total float on the affected sequence and the compensation event consumes 2 weeks of it, the assessment grants 0 weeks of extension, because the Completion Date hasn't moved.
I've seen this genuinely surprise senior commercial managers arriving from JCT backgrounds. They expect float to be a contractor-owned resource. Under NEC4, it isn't. Both parties can see it, and the mechanism consumes it before granting any EOT.
The practical consequence: a contractor who maintains programme float carefully is more exposed to concurrent delay arguments, because that float absorbs Client-risk events before they reach the critical path. A contractor who has already consumed their float through their own performance issues has, somewhat counterintuitively, a stronger case that the compensation event directly caused critical delay.
Contractor-Owned Risk Events
Under Clause 60.1, there are 21 categories of compensation event. Everything outside those 21 categories is a contractor risk. If the contractor is in delay due to a contractor risk event (say, a subcontractor resource failure), that delay does not entitle them to an extension of time or additional cost.
When a Client-risk event and a contractor risk event are both operating, the question becomes: which one is driving delay to completion? NEC4 doesn't provide a statutory answer. The contract requires an honest programme analysis to determine causation, and that programme analysis is only meaningful if the Accepted Programme is current and compliant.
Worked Example: Two Delays, One Programme
£45M Rail Infrastructure Package, NEC4 Option C
Completion Date: 14 November 2025. Accepted Programme accepted at week 12, showing 6 weeks of total float on the critical path.
The Project Manager issues a late design instruction (Clause 60.1(1)) affecting track formation on Section 4. The instruction is 4 weeks late against the required information date on the Accepted Programme. The contractor notifies CE-027 on 3 March 2025.
The contractor's earthworks subcontractor fails to resource Section 3 adequately. Section 3 feeds directly into Section 4. The subcontractor's under-resourcing is causing a 3-week delay on its own sequence.
At the point of the compensation event (3 March 2025), the Accepted Programme shows Section 3 on the critical path with a 3-week contractor-caused overrun. Section 4 was 2 weeks behind critical path (consuming part of the 6-week total float). The CE assessment under Clause 63 works from the Accepted Programme at the dividing date. The critical delay to completion is the 3-week subcontractor delay on Section 3. Even accounting for the 4-week instruction delay on Section 4, Section 4 cannot start until Section 3 completes, so the critical path is driven by the contractor risk event.
CE-027 grants additional Defined Cost for the design instruction work, but no extension to the Completion Date, because the critical delay at the dividing date is the contractor's own subcontractor failure. After week 28, when the subcontractor completes Section 3, the design instruction moves onto the critical path and CE-027 can be revised.
This is why concurrent delay under NEC4 is a moving target. It isn't decided at one point in time. It requires programme analysis at multiple assessment points.
The Case Law Position
NEC4 concurrent delay hasn't generated the volume of case law that JCT has, partly because the contract is newer and partly because many NEC4 disputes resolve in adjudication rather than litigation. But several key cases inform how these situations play out.
Henry Boot Construction v Malmaison Hotel (1999): While a JCT case, it established the principle that where there are two concurrent causes of delay, one an employer risk event, one a contractor risk event, the contractor is entitled to an EOT for the employer risk even if the contractor's own default would have caused equal delay. This "malmaison approach" does not directly apply under NEC4 (because NEC4 doesn't have a standalone EOT mechanism), but it informs how adjudicators think about causation.
North Midland Building v Cyden Homes (2018): The Court of Appeal confirmed that parties can contract out of the malmaison approach, and that a clause giving the contractor no EOT where concurrent delay exists is enforceable. Under NEC4, where the compensation event mechanism handles time prospectively against the programme, a similar result can occur without needing an express exclusion clause. The programme analysis determines causation, not a judicial default rule.
Walter Lilly v Mackay (2012): Akenhead J held that the contractor is entitled to an EOT if a relevant event (employer risk) caused delay, even if there was concurrent contractor default. Again a JCT case, but the factual analysis methodology, requiring an honest, critical-path-based assessment, is directly applicable to NEC4 programme analyses.
Saga Cruises BDF v Fincantieri SpA (2016): In an arbitration context, this case addressed the problem of concurrent delay where neither cause could be separated from the other. The principle that emerged, apportion where possible, but not where causes are genuinely inseparable, maps onto NEC4's requirement for honest programme analysis rather than mechanical apportionment.
There is no definitive NEC4 concurrent delay case that has settled the question at appellate level. Adjudicators apply their own experience and the programme evidence before them, which means the quality of your programme and records is everything.
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